1. Description of the invention
The present invention relates to a polymer composition for coatings with high refractivity, conductivity and transparency. More particularly, the present invention relates to the use of an inorganic sol, an amide solvent and a sulfonic acid group (SO.sub.3 H)-containing monomer dopant, in combination, in improving the refractivity, conductivity and transparency of a polythiophene-based polymer composition.
2. Description of the Prior Art
Representative of conductive polymers, polyanilines, polypyrroles, and polythiophenes, collectively called synthetic metal, have been suggested to be applicable for where conductivity and plastic properties are needed, such as electromagnetic wave-shielding materials, electrodes for secondary cells, transparent electrodes, etc, by virtue of their good conductivity and being easily polymerized. However, because they are very difficult to process in addition to being poor in stability against heat, atmosphere and UV light, only a few examples have been proved commercially successful.
Polyethylenedioxythiophene (PEDT), a conductive polymer, was disclosed as an antistaticity-imparting material, such as an antistatic coating, in U.S. Pat. Nos. 5,035,926 and 5,391,472, and its development is now being watched with keen interest because it is evaluated as solving the problems found in the above conductive polymers. With far superiority to polyanilines, polypyrroles and other polythiophene-based compounds in solubility, thermal and atmospheric stability, and resistance to UV light, PEDT can be used as a base material for coatings applicable to externally exposed parts to which the preexisting conductive polymers cannot be applied owing to their poor durability. Doped with a polymeric acid salt (e.g., polystyrene sulfonate), PEDT can be dispersed in water, and the dispersion is of good compatibility with lower alcohols, such as C1-C4 alcohol solvents, which are low in boiling point and ecologically favorable. The dilution in alcohols allows PEDT to be coated in various forms. Particularly, thin films made of such aqueous dispersions find numerous applications in cathode ray tube (CRT) glasses, plastic film surfaces, etc. Such water-dispersible PEDT polymers are now commercially available, representatively exemplified by Baytron P (Grade A4071) from Bayer.
To perfectly perform its characteristic functions, a coating for CRT external surfaces is required to be high in refractive index as well as conductivity. As being eligible for such a coating, inorganic thin films, such as ATO (antimontinoxide) and ITO (indiumtinoxide), have been used. A coating structure on a CRT glass panel is shown in FIG. 1. As shown in FIG. 1, the coating structure on a CRT glass panel 1 is typically composed of two layers: a high refractive, conductive layer 2 and a low refractive layer 3. Overlaying the conductive layer 2, the low refractive layer 3 is usually formed of SiO.sub.2. A display which employs an ATO thin film or an ITO thin film as the conductive layer 2 can reduce the reflected light intensity of incident light on the CRT glass surface through an interference operation. Because the ATO or ITO thin film has a refractive index of 1.6 or higher, the reflected light from the surface of the ATO or ITO thin film shows a phase difference from that from the surface of the low refractive layer. This reflection-attenuating function is indispensable to almost all CRTs for computer monitors, near to which are the eyes of the users.
Details about the materials and structures of the reflection-attenuating coatings can be referred to U.S. Pat. Nos. 5,681,885, 5,572,086, 5,652,477 and 5,523,469.
As for a conventional PEDT conductive coating solution, it has a refractive index of only 1.30-1.40 when being formed into a film, which is even lower than 1.54, the minimal RI value required for CRT glass panels. Thus, superior as they are to the inorganic coatings in conductivity and transparency, conventional PEDT coatings have not been applied to CRT external glasses because of being insufficient in refractive index.